Electro-optical addressing structures are employed in a variety of applications including video cameras, data storage devices, and flat panel liquid crystal displays. Such addressing structures typically include very large numbers of address locations arranged in an array. For example, a flat panel liquid crystal display configured in accordance with a high-definition television format would typically include at least two million address locations. The address locations would correspond to display elements or pixels that are arranged in about 1000 lines with about 2000 pixels each.
Adjacent pixels in such a display are closely spaced and have incidental capacitive couplings resulting from these small spacings. Such coupling between adjacent pixels will be referred to as "side-to-side" coupling. In addition, during operation of electro-optical addressing structures, the data drive signals for all the pixels in a row or column are typically carried on a common conductor adjacent the pixels. The electrical properties of the electro-optical addressing structures result in capacitive coupling among all the pixels in the column or row. Such coupling among all pixels in a column or row will be referred to as "front-to-back" coupling. These two types of capacitive coupling cause the data drive signal directed to a particular pixel to be carried to other pixels as incidental data signals or crosstalk.
For a display system, the crosstalk is image-dependent, i.e., it depends on the data drive signals present on the conductors and changes the voltage actually stored at a specific pixel. Crosstalk effects include an unpredictable gray scale that limits the number of achievable gray levels below the number necessary for acceptable video performance. A gray level is sensitive to small variations in the means square average voltage ("RMS") across a display element, and the crosstalk changes that voltage. It will be appreciated that gray scale in this context refers to the range of available light output levels in either monochrome or color display systems.
One type of electro-optical addressing structure used in flat panel liquid crystal displays employs an array of thin film transistors to address pixel locations. A driving method that reduces the image dependent crosstalk in such displays, known as Data Complement Drive ("DCD"), is described by Howard et al. in "Eliminating Crosstalk in Thin Film Transistor/Liquid Crystal Displays," International Display Research Conference, 230-35 (1988). DCD entails successively applying a data input signal and its complement to a row of address locations during a row addressing period.
In conventional addressing, a separate data drive signal, V.sub.i, is applied to each pixel of a row for a row address period. DCD entails applying the data drive signal V.sub.i to the pixels for one-half the row address period and then applying a separate data signal complement V.sub.i for the remaining one-half of the row address period. The data drive signal complement, V.sub.i, depends upon the data drive signal V.sub.i and is equal to the difference between a fixed level, V.sub.m, and the original data drive signal V.sub.i.
DCD does not adequately reduce all types of crosstalk effects in all addressing structures, particularly those having a relatively high susceptibility to crosstalk errors produced by side-to-side coupling. One such addressing structure is described in U.S. Pat. No. 4,896,149 of Buzak et al. for "Addressing Structure Using Ionizable Gaseous Medium", which is assigned to the assignee of the present application. The relatively high susceptibility to crosstalk errors produced by side-to-side coupling is believed to be a consequence of a physical configuration that positions address locations or pixels relatively far from an electrically grounded surface. The relatively large distance to the grounded surface allows the formation of incidental electric fields (i.e., crosstalk) among nearby pixels.
Another drive method for reducing crosstalk is known as the Return to Common Drive ("RTC"). RTC entails applying the data drive signal V.sub.i to the row of pixels for a first phase of the row address period and then applying a common voltage during the remainder of the addressing period. The common voltage is fixed and is independent of the data drive signals-the same common voltage is used for all columns and all lines of the display. This method effectively reduces side-to-side crosstalk, but is less effective in reducing front-to-back crosstalk.
Crosstalk may also be reduced, as described in U.S. patent application Ser. No. 07/854,145, which is assigned to the assignee of the present application, by using a two-phase addressing method in conjunction with a liquid crystal material that is insensitive to the frequency of the two-phase signals. Such frequency sensitive liquid crystals are not, however, suitable for all applications.